A power system or electrical network is said to be operating under steady-state conditions when there exists a balance between generated and consumed active power for the system. Power systems operating under steady-state conditions typically operate at or very near their nominal frequency.
Under certain circumstances, a power system can be disturbed such that it no longer operates under steady-state conditions. In that regard, power systems are subjected to a wide range of small or large disturbances during operating conditions. Small changes in loading conditions occur continually. The power system must adjust to these changing conditions and continue to operate satisfactorily and within the desired bounds of voltage and frequency.
A power swing condition can be the result of a disturbance that causes the power system to be removed from its steady state operating condition. Power system faults and their clearance, line switching, generator disconnection, and the loss or the application of large amounts of load are examples of system disturbances that can cause a power swing condition to occur in a power system. Upon the occurrence of a power swing condition, there exists an imbalance between generated and consumed active power for the system.
Depending on the severity of the system disturbance(s) and the actions of the power system controls during a power swing, the system may remain stable and return to a new equilibrium state, having experienced what is referred to as a stable power swing. However, severe power system disturbances can produce a large separation of system generator rotor angles, large swings of power flows, large fluctuations of voltages and currents, and eventually lead to a loss of synchronism between groups of system generators or between neighboring utility systems. This occurrence is referred to as an unstable power swing.
Power swings, whether stable or unstable, can cause undesirable results. In particular, power swings can cause the impedance presented to a distance protection to fall within the operating characteristics of the distance protection, away from the pre-existing steady-state load condition, and cause the distance protection to actuate an undesired tripping of a system transmission line. The undesired operation of the distance protections during a power swing can further aggravate the power system disturbance and cause system instability, major power outages and/or power blackouts. This can cause an otherwise stable power swing to become an unstable power swing. It will therefore be understood that distance protections preferably should not operate during stable power swings to allow the power system to establish a new equilibrium state and return to a stable condition.
During an unstable power swing, two or more areas of a power system, or two or more interconnected networks, lose synchronism, Uncontrolled tripping of circuit breakers during an unstable power swing condition could cause equipment damage and pose a safety concern for utility personnel. Therefore, it is imperative that the asynchronous system areas be separated from each other quickly and automatically in order to avoid extensive equipment damage and shutdown of major portions of the power system. During an unstable power swing condition, a controlled tripping of certain power system elements is necessary in order to prevent equipment damage, widespread power outages, and to minimize the effects of the disturbance.
In view of the above fact, if a fault occurs during a power swing, a distance protection performed by the distance protection should be able to operate reliably. The distance protection needs to detect the fault and select the fault phase quickly and reliably, under various power swing periods (for example, 0.1 s˜5 s), power angles when the fault occurs, and operating conditions (3-phase swing or single-pole open power swing, i.e. 2-phase swing).
In the conventional distance protections, for an asymmetrical fault during a power swing, it is normally detected by the presence of negative sequence and zero sequence components. The Chinese patent application No. 90211534.0 has disclosed such a method that set the criteria as |I2|+|I0|>m|I1|, where I0, I2 and I1 represent the zero sequence current, negative sequence current and positive sequence current respectively, and m is a coefficient between 0.5 and 1. Some other manufactures set the above criteria as |I2|>m|I1|, |I0|>n|I2|, where n also is a coefficient. Such unblocking method has different delays for different power angle between the equivalent systems at both terminals. If the fault occurs when power angle is small, the delay will be very short. However, under certain unfavorable conditions, the delay might be more than 30% of the power swing period.
For a symmetrical fault during power swing, it is normally detected with low power swing centre voltage (u cos φ). When the power swing centre voltage stays close to zero for more than a given period, such as 150 ms, 500 ms, etc., the distance protection is unblocked.
There are also some improved schemes for faster unblocking. For example, the Chinese patent application No. 03146340.1 has disclosed a method of distinguishing line fault with power swing based on change rate of measured resistance, which utilizes changing rate of measured resistance to unblock the distance protection.
However, the common problem existing in the above mentioned methods is that their response speeds are slow, especially for slow power swings. The long delays of power swing unblocking will slow down the operation speed of a distance protection, thus are unfavorable for the system reliability and device safety.